Under the new regulations for the reduction of hydrocarbon emissions from passenger cars in California and what are referred to as the green States in the United States of America, in which respect reference is made to low-emission vehicles (LEV), zero-emission vehicle (ZEV) and partial zero-emission vehicles (PZEV), the aim of the manufacturers of automobile tank venting systems having an activated carbon filter is to appropriately reduce the levels of emissions from the activated carbon filter. Multi-chamber systems or additional filter elements are used for main activated carbon filters, in order to reduce the emissions therefrom.
The actual activated carbon filter itself may be in the form of a canister with a loose activated carbon fill. The canister serves to trap the hydrocarbons which issue from the fuel tank of a motor vehicle while the vehicle tank is being filled. During operation of the vehicle it is back-flushed again with a defined amount of air and desorption takes place. The desorbed air is fed to the combustion air being passed to the engine. If for example the motor vehicle remains standing in the sun after such a desorption process has taken place, the vehicle heats up and therewith also the fuel tank together with its activated carbon filter. The heating effect has two consequences, as follows:
1. further hydrocarbons escape from the fuel tank and they again load the activated carbon filter, and
2. the activated carbon filter itself desorbs a given amount of hydrocarbons into the atmosphere, from the residual loading of the activated carbon filter, that is to say from the amount of hydrocarbons which, in the desorption operation, were not able to be completely flushed out of the activated carbon.
The above-mentioned new regulations known as LEV II and PZEV respectively provide that a passenger car or like vehicle as defined therein may only discharge a maximum of 0.5 g and 0.35 g respectively of hydrocarbons per day. That value is distributed to corresponding components by the individual automobile manufacturers, in which respect the emissions guideline value for the tank venting system of a passenger car is set at a maximum of 10 mg of hydrocarbon per day. It will be appreciated that the value of 10 mg per day is the level of emissions which may still be discharged to the atmosphere from the activated carbon canister on a daily basis.
Basically multi-chamber systems are used for the main canister in order to attain that aim. Such multi-chamber systems form additional diffusion paths for the gasoline vapor, and such paths result in a marked delay in the escape of gasoline vapor from the tank to the ambient atmosphere.
In this respect attention may be directed to U.S. Pat. No. 6,503,301 describing a multi-chamber system in which a chamber which is towards the atmosphere, besides the adsorbent, also contains a material with a high thermal capacity. That material is intended to receive the adsorption heat and thus increase the adsorption capacity of the chamber.
Reference may also be made to DE 199 52 092 C1 disclosing a multi-chamber system, wherein provided on the atmosphere side is an additional adsorptive volume which is intended to trap the residual emissions from the main activated carbon filter. Adsorption agents which may be used for that additional volume are silica gel, zeolites or ion exchangers. However, in comparison with the activated carbon, those adsorption agents involve only a very limited adsorption capacity and it is only with difficulty that they can be regenerated with air at ambient temperature. An activated carbon cloth or gauze which is also referred to therein can admittedly be satisfactorily regenerated but it only has a very limited adsorption capacity.
Consideration may be given here to DE 100 49 437 setting out a main activated carbon filter having an additional filter element, wherein the additional filter element has the property that it can be desorbed substantially more rapidly, in comparison with the main activated carbon filter, because the additional filter element is of a small size. A preferred material for that additional filter element is a non-woven fabric which is coated with activated carbon and which is rolled up to form a cylinder, with the gas flowing therethrough in the longitudinal direction of the cylinder. That additional filter element however suffers from the deficiency that it produces an additional differential pressure which is generally very high by virtue of the dense winding structure that it entails.
An SAE Paper 2001-01-0733 by Westvaco, dating from the year 2001, sets out a particularly advantageous structural configuration for an additional filter element. The unit therein employs honeycomb activated carbon filters which are distinguished by involving a particularly low differential pressure. In addition, by virtue of their small size, like also that described in above-discussed DE 100 49 537, those honeycomb activated carbon filters can be regenerated significantly more quickly than the main activated carbon filter.
Reference may be made to U.S. Pat. No. 6,537,355 disclosing a particular design of a honeycomb activated carbon filter, more specifically an activated carbon monolith, with a special sealing system.
The use of a honeycomb structure of activated carbon in a tank venting system is also described in U.S. Pat. No. 4,386,947. That specification explicitly refers to the good adsorption and desorption characteristics, by virtue of a uniform passage structure provided therein. Multi-chamber systems using such honeycomb structures are also described therein.
It will be noted that all the above-discussed systems disclosed hitherto are multi-chamber systems comprising at least two adsorptive volumes. The problem involved in the reduction of residual emissions however does not just entail providing a filter volume which has as good a desorbent effect as possible on the atmosphere side of the filter system, but rather the filter volume in question must afford quite specific properties so that it functions properly in the relevant situation of use. After that filter element has been flushed free it must have a residual capacity for hydrocarbons which can escape from the main canister constituting the main activated carbon filter. That residual capacity must be maintained when there is an increase in temperature from for example 20° C. to 42° C. In addition, if it transpires that the filter element has not discharged all previously adsorbed hydrocarbons again in the flushing process, the filter element is not to discharge them again due to a rise in temperature from for example 20° C. to 42° C.
That rise in temperature from 20° C. to 42° C. results from a prescribed test procedure specified by the California Air Research Board which can be referred to for brevity as the CARB, whereby a complete fuel tank venting system is subjected to a pre-ageing procedure in a defined manner in such a way that, in a predetermined number of operating cycles, it is loaded with hydrocarbons and repeatedly flushed clear again. The levels of emission are then recorded over a 2 or 3 day cycle. In the course of that test procedure, the entire fuel tank venting system is heated once per day from 20° C. to 42° C. and then cooled down again. In that procedure, on the one hand fuel vapors are caused to evaporate from the tank and are caught by the main activated carbon filter, while on the other hand hydrocarbons are desorbed from the main activated carbon filter and have to be trapped by the additional filter element.
The fuel vapors which are caused to evaporate from the tank during a tank refuelling operation and which are caught in the main activated carbon filter are fractionated there. The higher-boiling components are adsorbed the best while the low-boiling components such as n-butane, n-pentane, n-hexane and n-heptane are worst adsorbed. For that reason it is assumed that, after a given number of cycles, in the tank refuelling procedure, downstream of the main activated carbon filter, the above-listed substances from C4, namely n-butane, through C7, namely n-heptane, break through and load up the additional filter element. As no increase in temperature occurs during regeneration of the system by the flushing procedure and as the flushing times employed are generally very short, a residual loading also remains at the additional filter element. In that situation, with an increasing number of ageing cycles, n-heptane will increasingly collect at the additional filter element as a residual loading thereat, as that substance has the highest boiling point of the four hydrocarbons listed above. The other hydrocarbons with C4 through C6 are increasingly displaced. It is further assumed that, after a given number of ageing cycles, in the phase involving the increase in temperature to 42° C., it is only n-butane that still escapes from the main activated carbon filter as that substance has the lowest boiling point. Consequently the additional filter element must behave sorptively in such a way that the emission level is a maximum of 10 mg per day, in spite of the residual loading of the additional filter element, an increase in temperature from 20° C. to 42° C. and an additional n-butane loading stemming from the main activated carbon filter.
In this respect therefore the present invention seeks to provide a method of describing the sorption behaviour and characteristics of such an additional filter element so that, in conjunction with a main activated carbon filter such as an activated carbon canister, the additional filter element affords an overall system which at a maximum produces an emission of 10 mg of hydrocarbons per day.
Consideration may be given in this context to U.S. Pat. No. 6,540,815 in which an attempt has already been made to describe the adsorption behaviour of such an additional filter element. The procedure adopted therein however only involves looking at the adsorption isotherms. It is stated that, when the adsorption isotherms of the additional filter element are of a shallow configuration, it is possible to achieve a particularly good reduction in the overall levels of emission. The additional filter element is described in conjunction with the properties of the main activated carbon filter. The first filter element which is towards the tank side exhibits a steep gradient in respect of the isotherms when high levels of concentration are involved and between 5 and 50% of n-butane in air is said to have an incremental adsorption capacity of more than 35 g/l while the second filter element which is towards the atmosphere side, at high levels of concentration, exhibits a shallow configuration in respect of the isotherms and is said to have an incremental adsorption capacity of less than 35 g/l at between 5 and 50% of n-butane in air. The steep isotherm configuration of the first filter element describes the characteristics of typical known tank venting carbons.
Above-discussed U.S. Pat. No. 6,540,815 specifies the isotherms for the tank venting carbons BAX1100 and BAX1500 from Westvaco, which involve two typical activated carbons which are used in the tank venting sector. The activated carbon CNR115 from Norit and the activated carbon FX1135 from Pica which are both also used in relation to tank venting exhibit a similarly steep gradient at high levels of concentration and are markedly above 35 g/l in the specified concentration range. The shallow configuration in relation to the additional filter element is achieved either by the choice of a suitable activated carbon such as for example that described in Embodiment 3 of U.S. Pat. No. 6,540,815 or by dilution of a tank venting carbon in accordance with Embodiments 1 and 2 of U.S. Pat. No. 6,540,815.
However, classification as steep and shallow adsorption isotherms, depending on the respective situation involved, does not necessarily describe a properly functional system. It is possible to produce an additional filter element on the basis of a honeycomb body which has a very shallow adsorption isotherm, and it is possible in that way to achieve a reduction in the levels of emission of the main activated carbon filter. However, depending on the respective situation of use concerned, that reduction may still not be sufficient. It was further found that, with a very low level of emission in respect of the main activated carbon filter in the range of between 10 and 50 mg/day, an additional filter element which, at a high level of emission, still afforded a very good reduction, here affords only a slight reduction or indeed no reduction at all.
Reference may be made at this juncture to FIG. 1 showing the adsorption isotherms for three different additional filter elements in the form of a honeycomb body consisting of or including activated carbon. Examples 1 and 2 were produced in accordance with DE 101 04 882 while the third curve in FIG. 1 shows Example 2 from U.S. Pat. No. 6,540,815. All Examples which are discussed in the latter publication are illustrated in summarising form in Table 1 hereinafter:
TABLE 1B.E.T.BWC insurfaceTotal poreMicroporeMesoporeBWC in granulehoneycombareavolumevolume**volume***form+structure++Example 12000 m2/g1.3 cm3/g 0.1 cm3/g—  15 g/100 ml 1.7 gExample 21900 m2/g1.3 cm3/g 0.8 cm3/g0.92 cm3/g  10 g/100 ml1.85 gExample 32000 m2/g1.6 cm3/g0.85 cm3/g0.92 cm3/g  11 g/100 ml1.95 gExample 42000 m2/g1.0 cm3/g0.85 cm3/g0.33 cm3/g10.5 g/100 ml 2.3 gU.S. Pat. No. 6 540 815————— 2.4 g+++Example 2BWC denotes butane working capacity*from the nitrogen adsorption isotherm in accordance with Gurvitch**from the nitrogen isotherm in accordance with Barrett, Joyner and Halenda***from the mercury intrusion diagram calculated in accordance with the Washburn equation+measured in accordance with ASTM-D5228-92 with 100% n-butaneD5228-92, loading with 50% of n-butane in air, 0.1 l/min desorption with 22 l/min for 15 minutes+++calculated from the information in Table 1 of U.S. Pat. No. 6 540 815.
It is possible to clearly see the differences in pore distributions and the internal surface areas of the activated carbons used. Equally clear are the differences in the n-butane working capacity indicated as BWC. The operation of determining the butane working capacity on the activated carbon granules was carried out in accordance with ASTM-D5228-92 with 100% of butane. The operation of determining working capacity on the honeycomb structures was also implemented on the basis of ASTM-D5118-92. Loading was effected with 50% of n-butane in air at 0.1 l/min until a breakdown of 5000 ppm, followed then by desorption with 22 l/min for 15 minutes.
FIG. 1 clearly shows that all three Examples illustrated exhibit a flat adsorption isotherm which applies in respect of the virgin condition, which in the region between 5% and 50% in the gradient are markedly below 35 g/l. Examples 1 and 2 therefore, like Example 3 of U.S. Pat. No. 6,540,815, should result in a reduction in the levels of hydrocarbon emissions. That fact is firstly made clearly by FIG. 2 showing the levels of emission of an activated carbon canister with and without the filters from Examples 1 and 2. The activated carbon canister itself has implemented markedly more than 100 cycles with fuel. For the measurement procedures, it was firstly loaded, with and without an additional filter element respectively, for three cycles with n-butane with a concentration of 50% in air until breakdown of 2 g, and was then backflushed with 300 l. After a rest period of 6 hours two heating cycles were started, during a period of two days. FIG. 2 clearly shows the reduction in the levels of emission by the use of an additional filter element. The filters from those two Examples are then subjected to measurement in tank venting systems using the same procedure but with markedly lower emissions in respect of the main activated carbon filter. The results from those two experiments are shown in Table 2 hereinafter.
TABLE 2Emissions after 20 cycles preageingof the overall system with fuelActivated carbon filterActivated carbon filterwithout additional filterwith additional filterExample 1  33 mg/day  31 mg/dayExample 27.95 mg/day10.3 mg/day
It will be seen that no reductions in emission were achieved in both cases by virtue of the additional filter. In spite of the shallow nature of the adsorption isotherms of the additional filter elements, it was not possible to reduce the levels of emission from the main activated carbon filter.